It's a debate that's gone on for years - and now it's finally coming to a head.

The compact M4 carbine - a shortened version of the M16 - that is now standard issue for most Army troops, some Marines and other specialized units is facing increased criticism because of its tendency to malfunction with even the minutest exposure to the elements.

Some ground communities, including special operations forces, have begun to sideline the M4 in favor of newer, gas-piston operated variants such as the Heckler & Koch-manufactured 416 and the FNH-built Special Operations Forces Combat Assault Rifle, or SCAR

In a routine acquisition notice March 23, a U.S. Special Forces battalion based in Okinawa announced that it is buying 84 upper receiver assemblies for the HK416 to modify their M4 carbines. The M4 fires using a system that redirects gas from the expended round to eject it and reload another. The 416 and SCAR use a gas-operated piston that physically pushes the bolt back to eject the round and load another.

Carbon buildup from the M4's gas system has plagued the rifle for years, resulting in some close calls with Soldiers in combat whose rifles jammed at critical moments.

According to the solicitation for the new upper receiver assemblies, the 416 "allows Soldiers to replace the existing M4 upper receiver with an HK proprietary gas system that does not introduce propellant gases and the associated carbon fouling back into the weapon's interior. This reduces operator cleaning time, and increases the reliability of the M4 Carbine, particularly in an environment in which sand and dust are prevalent."

The 416 is used by the Army's elite Delta Force, and a recent Army Times investigation showed the service's top equipment buyers ignored data from the spec ops community showing the M4 had fundamental flaws. Enamored by the development of futuristic weapons such as the XM29 and, later, XM8 - neither of which were ever fielded - the M4 stayed in the hands of Soldiers deploying to hot, dusty, austere environments like Iraq and Afghanistan.

The Army would prefer to wait for the development of a new rifle firing an airburst, round - essentially leaping ahead of today's technology. But that innovation has been hard to find in the right weight class.

An Army spokeswoman for Program Executive Office Soldier, based at Fort Belvoir, Va., said in a statement the Army isn't buying into SOF's argument.

"At this time PEO Soldier is not procuring and does not have plans to procure the 416," said Army spokeswoman, Erin Thomas, in an email statement.

But special operations forces sometimes work outside the "Big Army" procurement system, so they can grab the best gear quickly.

"The elimination of the gas tube ... means that the M4 will function normally even if the weapon is fired full of water without first being drained," the justification for the 416 assembly buy states. "There isn't another company that offers these features in their products. It is a practical, versatile system."

Army weapons experts have been tinkering with new weapons designs, such as the HK-built XM8. Its modular design, rugged construction and accuracy intrigued many in the Army - and other services. But in 2005, the Army abandoned the XM8 after spending $33 million - though the Natick Soldier Systems Center has been looking at a shortened version of the XM8 as a personal defense weapon for officers and armored vehicle crews.

So far, however, the Army is unwilling to buy what the special operations community believes is a clearly superior system and is still spending money looking for another technology while Soldiers use what many say is an inferior weapon in harsh combat conditions.

"The Infantry School at Fort Benning, Georgia is currently conducting a Capabilities Based Assessment to determine future Army needs," Thomas said in the statement, declining to elaborate.